The arbor or cutter drum of a woodcutting or chip-producing machine usually is a cylindrical body which is rotated about its longitudinal axis and which is provided on its periphery with a plurality of longitudinally extending grooves in which straight-edged blades are clamped with their edges lying on a cylindrical orbit centered on the rotation axis of the arbor. It is very important that the straight edge of each blade lie exactly parallel to the arbor's rotation axis and directly on the orbit in order to produce wood chips of uniform dimensions.
The grooves in the drum may be of a predetermined depth and the blades are of a predetermined width so that in theory they need merely be seated in these grooves so as to have their cutting edges properly positioned. In such a system, however, any deformation of the cutter drum misaligns the blades. Similarly if the blades wear and/or are reground, their width changes so that they must be reset painstakingly by hand by a skilled worker using a micrometer gauge or the like and even then any irregularities in the shape of the drum will be reflected in the setting of the blade edges.